In consideration of environmental problems in the automotive industry, in particular, weight reduction of automotive bodies has progressed in recent years, which requires metal sheet materials used for automotive bodies to have smaller thickness while having high strength and rigidity. In view of these requirements, high-strength steel sheets have gradually been adopted as a metal material.
When, however, a high-strength steel sheet is used, a phenomenon, specifically, a large amount of springback occurs in the steel sheet during forming. If a large amount of springback occurs in a sheet, the sheet is described as having a defective shape and fails to be assembled by joining or by other methods. Thus, a springback reduction method is an extremely important technology and various different methods have been developed.
Patent Literature 1 is taken as an example of the springback reduction method. Patent Literature 1 discloses a die for press forming with which a metal material is processed so as to have a hat-shaped cross section without deep drawing and a compressive force is applied to a vertical wall portion of the metal sheet material immediately after the bending step is finished.
In addition, a method for analyzing a cause of springback before performing the springback reduction method is disclosed in, for example, Patent Literature 2. Patent Literature 2 discloses an analysis method for press forming with which a portion of a press-formed part that highly contributes to springback is accurately calculated in a short period of time. As specific measures against springback, Patent Literature 2 describes optimization of press forming conditions such as the shape of a die of press forming, the shape of a blank, or the way how a drawbead is inserted ([0031] of Patent Literature 2).